The royal couple chose Fiona Cairns, a British pastry chef and graphic designer famous for imaginative ideas, to make the cake that they and their guests will enjoy at the reception. Cairns has created pastries for other famous faces, such as Bono, Sir Paul McCartney and Pink Floyd. Her first cookbook, "Bake and Decorate," was released in March.

In an interview with "Nightline," Cairns explained that she and her baking team had put in hundreds of hours planning, designing and baking this wedding cake of such high prominence.

She said that Kate had been "very involved" in the cake selection from the start.

"She briefed us very closely right at the beginning," Cairns said. "So it's really her creation, our interpretation. ... We made a few mockups ... with piping ideas and flowers and submitted those, and then Catherine sent them back with her notes: what she liked, what she didn't like."

Cairns explained that the outside of William and Kate's cake has a floral motif, each with a special meaning.
"There's something called the Language of Flowers," Cairns said. "Daisies symbolize innocence; acorns symbolize strength."

She said Kate sent a list of 17 flowers -- symbolizing love, marriage and happiness -- that she wanted on the cake. "There's even a Sweet William," she said.

Cairns' assistants have spent weeks crafting the flowers out of sugar paste. "At the time, I thought, 'How on earth are we going to make heather and lavender?'" she said.

Cairns admitted there had been some sleepless nights making the royal wedding cake but not many worries about what might go wrong. The pastry chef and her baking team are well prepared.

"If you're thinking that we might break something, we do have extra parts," she said. "We've thought of that."

Underneath the flowers and delicate white and cream hues is a traditional English fruitcake.

Given the weather forecast for the big day, there may be lashings of rain as well. Tyler remains undaunted. Kate will not have a soggy fruitcake on her big day: "I'm very confident she won't. We've perfected this," she said.

After it was baked, the cake spent weeks in the "maturation room," suffused with aromas of alcohol and fruit. Since then it has been under guard behind palace walls.

Even Americans would agree: This is one fruitcake one wouldn't want to regift.